The present invention relates to a handsfree unit corresponding to a mobile radio communication terminal of portable telephones and car mobile telephones, which can be used by selectively switching the analog mode and the digital mode, and allowing a loud communication to be made without having a communication terminal body in hand, and also relates to a dual-mode mobile radio communication apparatus using the handsfree unit.
Recently, a handsfree unit corresponding to a mobile radio communication terminal which can be used by selectively switching the analog mode and the digital mode or what is called a dual-mode apparatus, has been developed.
As is well known, a handsfree unit employs a voice switch system for introducing a constant amount of attenuation to the transmission side or reception side in response to the communication state, or uses an echo canceling circuit, in order to prevent generation of the howling phenomenon and reduce the sound which is input from a loud-speaker to a microphone.
However, if a handsfree unit employing the voice switch system is used for a dual-mode apparatus, when attenuation enough to prevent the howling phenomenon in the analog mode is set, signal processing becomes complicated in the digital mode as compared with the analog mode and the signal delay occurs and, therefore, the sound input from a loud-speaker into a microphone cannot be sufficiently reduced.
If a handsfree unit comprising an echo canceling circuit is used for the dual-mode apparatus, sufficient advantages can be obtained in both the analog mode and the digital mode, but the manufacturing cost is increased as compared with the handsfree unit employing the voice switch system.
In a small indoor room of a car mobile, the sound which is output loud from a loud-speaker is input directly to a microphone and also input after reflection on walls inside the car mobile and, therefore, the above-described problem is remarkably serious.
In a conventional handsfree unit, if the voice switch system is employed, the sound input from the loud-speaker to the microphone cannot be sufficiently reduced in the digital mode. In addition, if the echo canceling circuit is used, the manufacturing cost is remarkably increased.